1. Field of the Invention
This invention relates to a process for preparing a low-crystalline soft or semi-hard copolymer of high transparency by copolymerizing ethylene, propylene and a straight-chained .alpha.-olefin having not less than four carbon atoms.
2. Description of the Prior Art
As a soft or semi-hard resin, polyvinyl chloride has heretofore been known, and in recent years copolymers consisting primarily of ethylene have also come to be known. As to a soft or semi-hard copolymer consisting primarily of propylene, however, various proposals have been made but have not been put to practical use yet.
Recently there has been proposed a process for preparing a soft or semi-hard copolymer by random copolymerization of propylene and butene-1 (see Japanese Patent Public Disclosure No. 79984/78). This process, however, involves a problem undesirable in point of production cost such that the expensive butene-1 must be used in a large amount.
As to a process for preparing a terpolymer of ethylene, propylene and an .alpha.-olefin having not less than four carbon atoms, there are known processes, for example, those disclosed in Japanese Patent Public Disclosures Nos. 79195/76 and 26883/78. All these processes, however, employ a catalyst comprising the combination of a TiCl.sub.3 -containing component and an organoaluminum compound to prepare a crystalline terpolymer. They are disadvantageous in that a soft or semi-hard copolymer is not obtainable.